Methyl side groups

In natural rubber, the cross-linking of these radicals is hindered because of the bulkiness of the methyl side group. Consequently, these radicals prefer to disproportionate and cleave. This reduces the molecular weight and natural rubber softens on ageing. 

The properties of the polycarbonate of bisphenol A are directly related to the structure of the polymer. The molecular stiffness associated with this polycarbonate arises from the presence of the rigid phenyl groups on the molecular chain or backbone of the polymer and the additional presence of two methyl side groups. The transparency of the material arises from the amorphous (noncrystalline) nature of the polymer. A significant crystalline structure is not observed in the polycarbonate of bisphenol A because intermolecular attractions between phenyl groups of neighboring polymer chains in the melt lead to a lack of flexibility of the chains that deters the development of a crystalline structure. 

FIGURE 6.12 Mechanism of formation of benzyl nitrate and benzaldehyde from reaction from the methyl side group of toluene. 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

Lewandowski and Ollis carried out thermodynamic calculations for the reaction of chlorine radicals with branched (toluene, xylenes) and unbranched (benzene) aromatic contaminants [51], On a branched aromatic contaminant, abstraction of the hydrogen atom on a methyl side group by a chlorine radical. 

Polyimides containing N-phthalimido and methyl side groups were prepared by the interaction of... 

A structure similar to that just considered is also present in the trans-1,4-butadiene/lmns-1,4-pentadiene-1,3 copolymers, although, in this case, the presence of methyl side groups increases the lateral distance among the packing chains (see the preceding section (25)). 

In contrast, the electrical conductivity tends to become higher with a lower silole content, reaching up to 2.4 S cm-1 upon doping with iodine. The bulky (t-butyldimethylsiloxy)methyl side groups in the silole unit are oriented out of the n-conjugated polymer plane and hence hinder the interaction between the polymer chains, which is known to be one of the key factors for high conductivity values174. 

The activation energy t for the cyclic hydrocarbons on Ni—AI2O3 are practically independent of the number of methyl side groups (Nos. 1-3). For amines on Ni and Pd (Nos. 23-39) the are also quite constant. 

Model compounds of 627, 629, and 630 containing methyl side groups were designed for the calculations. The results revealed that all the... 

FEP, a copolymer of tetrafluoroethylene ( IFF) and hexafluoropropylene (HFP), is essentially PTFE with an occasional methyl side group attached. The methyl groups have effect as defects in crystallites and therefore reduce the melting point. These side groups also impede the slipping of the polymer chains past each other, thus reducing the cold flow. 

Lattice substitution requires that the incorporated impurity be of similar size and function to the primary crystallizing species. In other words, the impurity must fit into the lattice without causing significant dislocations. An example of such a system is found in the crystallization of L-isoleucine in the presence of trace quantities of L-leucine. The two species have similar molecular structures, differing only by one carbon atom in the position of a methyl side group. In this system, the incorporation of L-leucine in L-isoleucine crystals is proportional to the concentration of L-leucine in the mother liquor. Moreover, the shape of the recovered crystals changes as the content of L-leucine in recovered crystal increases. 

A similar observation of reduced crystallinity caused by methyl side groups was noted by Yu and Evans (17), who find that the introduction of 2-methyl substituents in the 4,4 position in the heptanediamine yields an amorphous polyterephthalamide. The corresponding polyamide of... 

The three methyl groups have no substantial influence on the glass temperature of our polyamides, but they do increase it a little even in the last sample in Table II despite the higher weight content of the aliphatic segment. In both cases we have three methyl side groups on 14 linear chain segments. 

Substitution of methyl side group by phenyl one at silsesquioxane atom of silicon induces abrupt decrease of hydrogen chloride liberation rate from 20 -25% (n = 2. R = Ph) to 5% (n = 4, R = Ph), which is displayed by curves 4 and 5 in Figure 1. 

Although the molecular origin of these unusual properties is still not known definitively, a number of suggestions have been put forward. One explanation involves low intermolecular interactions, and another involves the very high rotational and oscillatory freedom of the methyl side groups on the polymer (8). Still another theory focuses on the chain s very irregular cross section, which is very large at the substituted Si atom and very small at the unsubstituted O atom (37). 

The influence of the butyl side groups on the tensile properties of the extrudates was even stronger than that of the methyl side groups. This is demonstrated in Table 3 for extrudates where the nominal draw ratios were 9 and 15. The strong decrease of the axial room temperature modulus with increasing size of the side groups went parallel with the increase of the amount of die swell. 

FIGURE 23.6 The structures of (a) isotactic, (b) syndiotactic, and (c) atactic polypropylene. In these structures the purplish-pink spheres represent —CH3 (methyl) side groups on the long chain. 

Example An example of an atactic polypropylene chain with randomly oriented methyl side groups ... 

Similar studies in aqueous solutions of polyCacrylic acid), PAA, i.e., variation of vs, pH are shown in Figure 3B. A smaller increase in with Au 0 fluorescence was observed around pH 3-4 than in the case of PMA, however, it is still 6-7 times larger than that in water. The data again indicate that due to the lack of methyl side groups i.e. lack of hydrophobic interaction PAA forms much looser coils at low pH than PMA, with smaller effects of the conformational transition on the photophysics of Au 0. 

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